corrosion study of stainless steels in peracetic acid bleach media

International Journal of Research and Innovations in Science and Technology©SAINTGITS College of Engineering, INDIAwww.journals.saintgits.orgResearch paper

Corrosion Study of Stainless Steels in Peracetic AcidBleach Media With and Without Chloride and Chelant

Rohtash1*, Ajay K. Singh2, Rajendra Kumar3

1 Assistant Prof. MRK Institute of Engineering & Technology, Rewari, India,2 Professor of Physics Department of Paper Technology, IIT Roorkee, India

3Prof & Head, Department of Physics, Gurukula Kangri Vishwavidyalaya, Haridwar, India.*Corresponding author E-mail: [email protected]

Copyright © 2014 IJRIST. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The paper industries are adopting non-chlorine containing chemicals e.g. peroxide, ozone, peracids etc. as alternate ofchlorine based bleach chemicals e.g. chlorine and chlorine dioxide etc. with the aim of eco-friend atmospheres.Changeover to the new chemicals in the bleaching process is likely to affect the metallurgy of the existing bleach plantsdue to change in the corrosivity of the media. Accordingly, corrosion investigations were performed in a peracid namelyperacetic acid to test the suitability of austenitic stainless steels 654SMO, 265SMO, 2205, 317L and 316L. Theperformance of above stainless steels was evaluated through long term immersion tests and Electrochemicalpolarization measurements in peracetic acid (PAA) bleach media at pH value 4 maintaining concentration 0.2 % asactive oxygen along with three chloride levels 0, 500 and 1000 ppm in pulp-free laboratory. To study the effect ofcorrosion inhibitors with extending limit of chloride in liquors, measurements were also made with two types ofchelants- EDTA & MgSO4. The results showed that corrosivity of PAA reduced by addition of chelant while increasedwith concentration of Cl¯. The results also exhibited that EDTA is better inhibitor than MgSO4.

Keywords: Bleach plant corrosion, chelants, electrochemical reactions, peracetic acid, stainless steel, material selection.

1. Introduction

Peracetic acid is considered as effective for bleaching like peroxide and ozone but this is preferred over the later twoalternatives on account of their superiority with regard to associated safety hazards, ingredient costs and effluent loads.Peracetic acid has not been adopted by Indian Paper Industry on considerable scale up to now but these are the futuristicbleach chemicals. The work in this article mentions the corrosion effects on stainless steels in peracetic acid bleachsolution with and without chloride as well as effects of corrosion inhibitors. With the use of newer chemicals, theircorrosive effect has been studied. There are very few studies related to corrosivity in peracids solutions, Pehkonen et al[7] studied corrosion of SS in ozone and PAA. Been [5] studied the effect of calcium as inhibitor and Varjonen et al[21] studied titanium with MgSO4 inhibitor in H2O2. Recently, some studies by Singh et al [9, 10 &11] has appeared oncorrosivity of peracids bleach media. A study on SS has been done by Singh R & Singh A. K. on SS using chelants butin peroxide media [13]. Another study has been done in same bleach media on cold rolled steel by Qing Quad, Jun Zhouet al [16]. In present study corrosion investigations were performed in peracetic acid to test austenitic stainless steels654SMO, 254SMO, 316L, 317L and a duplex stainless steel 2205.

Electrochemical polarization and long term immersion tests were performed in the bleach solutions of PAA with threechloride levels. Corroded steels were analyzed for uniform corrosion pitting, crevice corrosion and welded relatedattack. The results, when compared to other corrosion studies in peracids media [10 & 11] showed peracetic acid to bemore corrosive than Caro’s acid in all similar test conditions of chloride levels in peracetic media with and withoutchelant. On the basis of results from this study, an attempt has been made to suggest the appropriate steel materials for

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handling different in solutions for bleaching in paper industry. So, this paper report on investigations were performed inperacetic acid solutions to said austenitic stainless steels (Table1) with chloride level 0, 500 and 1000 ppm and chelantEDTA (0.2% of pulp) , MgSO4 (0.25% of pulp) in laboratory simulating all the conditions of bleach plant of paperindustry as appropriate bleaching.

2. Experimental

2.1. Material

For Corrosion study, Test samples from plate samples of austenitic stainless steels- 6% Mo (654SMO, 254SMO), 317L,316L and a duplex stainless steel 2205 were selected for the weight loss tests and electrochemical tests. Thecomposition of these stainless steels is shown in Table 1. All the SS test samples were as received in the solutionannealed condition as per ASTM specification A 240 [14]. Before exposure, the coupons were polished until a mirrorfinish surface is obtained, for achieving this state polishing machine was used. Polishing and finishing were done byabrading the specimen on emery papers with grit sequence from coarser to finer i.e. up to 800, 1000 grit size and thensubjected to finishing papers 1/0, 2/0 and at last 4/0. Finally the samples were degreased using acetone solution, andweighed. For the electrochemical studies, coupons of 1 cm2 were embedded in a mould of epoxy resin and an electricalconnection was established via a copper wire. The polished test metals were degreased as described above. All thechemicals used during characterization of bleach solutions, were AR grade.

Table 1: Composition of stainless steels plate samples (%)

ALLOY C Ni Mn P Cr Mo S Cu N Si316L 0.020 10.87 1.69 0.030 17.44 2.16 0.030 0.31 0.04 0.69317L 0.016 13.44 1.75 0.026 18.80 3-7 0.004 0.42 0.047 0.502205 0.020 5.480 1.45 0.026 22.25 3.08 0.002 0.21 0.15 0.52

254SMO 0.009 18.00 0.44 0.028 20.10 6.15 0.001 0.74 0.20 0.31654SMO ≤ 0.02 21-23 2-4 ≤0.30 24-25 7-8 0.005 0.3-0.60 0.50 0.50

Table 2: Chemical composition of PAA bleach solutions.

Solution NameChemical Charges

Conc.(as % AO) pH Cl¯(ppm) ChelantPA1 0.2 ±0.04 4.0 ±0.05 0 withoutPA2 0.2 ±0.04 4.0 ±0.05 500 ±0.50 withoutPA3 0.2 ±0.04 4.0 ±0.05 1000 ±0.50 without

PA1CI 0.2 ±0.04 4.0 ±0.05 0 EDTA =0.2 ±0.02%PA1CII 0.2 ±0.04 4.0 ±0.05 0 MgSO4=0.25±0.02%PA2CI 0.2 ±0.04 4.0 ±0.05 500±0.50 EDTA =0.2 ±0.02%PA2CII 0.2 ±0.04 4.0 ±0.05 500±0.50 MgSO4=0.25±0.02%PA3CI 0.2 ±0.04 4.0 ±0.05 1000 ±0.50 EDTA =0.2 ±0.02%PA3CII 0.2 ±0.04 4.0 ±0.05 1000 ±0.50 MgSO4=0.25±0.02%

2.2. SolutionsTable 2 shows the test conditions and composition of the solutions of PAA test media. For both the long termimmersion test and the electrochemical polarization test, the stainless steel samples were exposed to three peracetic acid(PAA) solutions namely PA1, PA2 and PA3 according to desired Cl¯ levels 0, 500 and 1000 ppm respectively. pHvalue of these solutions was adjusted and kept in the range 4.0 ±0.05 by adding required amount of sodium hydroxide.Six solutions were prepared on adding chelants in PAA, namely PA1CI, PA1CII, PA2CI, PA2CII and PA3CI, PA3CIIhere CI refer as chelant I namely EDTA while CII refer chelant II namely MgSO4 (Table 2). All the solutions of PAAwere prepared as per described Amini and Webster [1]. Accordingly, peracetic acid (CH3COOOH) was prepared byadding glacial acetic acid to hydrogen peroxide in one molar ratio. The mixture was then warmed to 45°C and held atthis temperature for two hours, and stored overnight in a refrigerator to allow the mixture to come to equilibrium.Bleach solutions (Table 2) containing Cl¯ were prepared by adding the calculated quantity of sodium chloride neededaccording to desired Cl¯ levels and adding to the solutions calculated values of both the selected chelants maintainingthe reaction temperature at 30-35°C. The concentration of PAA, added chloride, pH value, concentration of chelant andother chemical charges in test media were maintained by periodic test and titration during the whole long termimmersion (weight loss) and electrochemical measurements.

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International Journal of Research and Innovations in Science and TechnologyVolume 1 : Issue 1 : 2014

3. Corrosion Tests

3.1. Long term Immersion tests

For weight loss test, the prepared stainless steel coupons were exposed for six months at room temperature (27°Caverage) each coupon was immersed in the solutions of peracetic acid as per described in Table 2 with serrated washersto initiate crevice corrosion. During the test, the percent active oxygen (AO) and pH value of the test solutions weremonitored and maintained each third day. While pH did not change much (±0.04), decrease in percent AO wascompensated to keep it within limit, as per Table 2 by adding requisite amounts of peracetic acid. After the six monthexposure, the corroded coupons were cleaned mechanically as per as per ASTM G1-72 [14]. Further tested couponswere cleaned by treating them with a cold solution of concentrated hydrogen chloride with 50 g/L stannous chloride and20 g/L antimonies chloride [17]. The coupons were then weighed to determine corrosion rate in mils per year as;

6 3.45 10 WCorrosion rate mpyDAT

(1)

Where W =weight loss [gm],D=density of metal in [gm/cm3]A = area [cm2]T =exposure time [hours]

To estimate the extent of localized attack the corroded samples were viewed under the microscope. The extent of pittingwas estimated by measuring maximum pit depth on the surface of cleaned coupons using a stereo microscope (Olympus)and an optical microscope (Leica Q500MC).

3.2. Electrochemical testsElectrochemical polarization tests were carried out using Radiometer “Voltalab” Electrochemical Laboratory ModelPGZ301of Radiometer France supplied with compatible corrosion measurement software (Volta Master-4). A saturatedcalomel electrode (SCE) was used as the reference electrode, graphite rods as the auxiliary, and the test specimen wasthe working electrode within corrosion cell. The tests conducted were open circuit potential (OCP), potentiodynamic(anodic) polarization, cyclic polarization and potentiostatic with the help of software Volta master 4. All theexperiments were carried out at 27°C. Figure 1 shows some representative curves of different stainless steels obtainedthrough conducted electrochemical tests. Tafel plots were used to evaluate corrosion potential (Ecorr), Icorr and βa, βc,(anodic and cathodic Tafel slopes) and hence corrosion rate. Other polarization techniques give an idea aboutpassivation range and possibility of pitting or crevice corrosion. Electrochemical parameters derived from these curvesare presented in Table 4. All the potentials referred in paper are with respect to saturated calomel electrode. Eachelectrochemical test was repeated to verify the reproducibility of the results. The electrochemical tests described abovewere conducted in the bleach solution having conditions mentioned above; by using a potentiostat and polarization cellhaving five necks meant for a working electrode, two counter electrodes, a reference electrode saturated calomelelectrode (SCE) as reference electrode and for gas purging. Open circuit potential (OCP), Corrosion (Ecorr.), pittingpotential (Ep), and repassivation potential (Ec) were evaluated from these tests. Figure 1 shows some representativecurves of different stainless steels obtained through conducted electrochemical tests.

Table 3: Long term immersion test results (corrosion rate and maximum pits’ depth)

NT= Not tested in case of with chelant for 6% Mo SS (654SMO, 254SMO)

SolutionMetal

Without Chelant With ChelantPA1 PA2 PA3 PA1CI PA1CII PA2CI PA2CII PA3CI PA3CII

Corrosion rate (mpy)316L 0.11 0.23 0.28 0.056 0.056 0.082 0.09 0.13 0.18317L 0.09 0.12 0.14 0.021 0.026 0.025 0.07 0.051 0.0832205 0.06 0.12 0.15 0.022 0.029 0.016 0.019 0.050 0.080

254SMO 0.04 0.061 0.062 NT NT NT NT NT NT654SMO 0.035 0.043 0.045 NT NT NT NT NT NT

Maximum Pit’s depth(µm)316L 87 153 193 90 105 120 133 152 165317L 73 142 165 60 48 88 103 130 1462205 55 139 173 42 34 72 95 130 141

254SMO 38 37 64 NT NT NT NT NT NT654SMO 31 40 53 NT NT NT NT NT NT

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Table 4: Electrochemical test results (corrosion parameters, pot. in mV)

Solutions→ PA1 PA2 PA3 PA1CI PA1CII PA2CI PA2CII PA3CI PA3CIIMetal↓ CP

316L

OCP 260 242 138 313±17 266±21 280±11 258±21 240±15 190±11Ecorr 282 232 149 214±3 198±6 240±23 210±14 128±8 155±3Epass 761 520 367 818±15 785±9 634±21 610±09 480±23 470±22Ep1 1021 807 600 1060 1027 915 902 789 834Ep2 1005 630 505 1145 1113 801 750 595 550Ep3 700-750 700-750 600-650 800-850 800-850 750-800 750-800 700-750 650-700Ec 932 141 54 **** **** **** 120 133 20

CR (mpy) 0.31 0.38 0.46 0.18 0.21 0.30 0.38 0.40 0.44

317L

OCP 258 172 132 330±7 270±15 260±21 220±10 170±4 157±5Ecorr 290 250 190 256±10 204±18 230±17 250±23 130±22 132±3Epass 830 755 532 946±11 941±23 835±11 805±19 927±11 771±10Ep1 1025 997 744 1210 1221 1153 1133 958 816Ep2 1053 1011 980 1127 1100 1154 1098 1135 1100

Ep3 900-1000

900-1000 700-750 1200-

12501150-1200

1050-1100

1000-1050

1050-1100

950-1000

Ec 1028 **** **** **** **** ** ** ** **CR (mpy) 0.23 0.26 0.35 0.12 0.15 0.18 0.24 0.20 0.30

2205

OCP 325 300 255 352±5 320±22 340±21 324±15 270±14 293±15Ecorr 291 271 210 323±06 336±10 350±16 312±21 225±21 160±15Epass 880 801 738 1042±1 1000±9 1012±17 980±11 997±31 869±13Ep1 1120 1075 967 1220 1152 1112 1076 1052 1017Ep2 1044 1056 1012 1131 1114 1096 1034 1047 1027

Ep3 1100-1150

1100-1150

1000-1050

1250-1300

1200-1250

1200-1250

1150-1200

1100-1150

1100-1150

Ec 985 993 912 **** 1020 1010 990 1080 911CR (mpy) 0.18 0.25 0.33 0.12 0.13 0.20 0.23 0.25 0.26

254SMO

OCP 376 338 300 NT NT NT NT NT NTEcorr 245 200 210 NT NT NT NT NT NTEpass 922 836 788 NT NT NT NT NT NTEp1 1159±0 1025 998 NT NT NT NT NT NTEp2 1041±1 1076 1040 NT NT NT NT NT NT

Ep3 1150-1200

1050-1100

1000-1050 NT NT NT NT NT NT

Ec 963 1042 949 NT NT NT NT NT NTCR (mpy) 0.12 0.12 0.17 NT NT NT NT NT NT

654SMO

OCP 445 372 360 NT NT NT NT NT NTEcorr 232 217 245 NT NT NT NT NT NTEpass 962 843 814 NT NT NT NT NT NTEp1 1165±1 1095 1057 NT NT NT NT NT NT

Ep2 1120±2 1067 1077 NT NT NT NT NT NT

Ep3 1200-1250

1150-1200

1100-1150 NT NT NT NT NT NT

Ec 1016 1008 961 NT NT NT NT NT NTCR (mpy) 0.11 0.13 0.19 NT NT NT NT NT NT

CP (Corrosion Parameter),CR (Corrosion rate from anodic Pol. Test in mpy),OCP (Open circuit potential),Epass (Passivation range) i.e. Ep1-Ecorr,Ecorr = Corrosion Potential,Ep1 Pitting potential from anodic polarization,Ep2 pitting potential from cyclic polarization,Ep3 pitting potential from potentiostatic measurements,Ec =Repassivation potential, all the potentials in mV,Pol = Polarization,**** Cases in which cyclic curve does not cut at all during reverses direction (least resistance to crevice corrosion),**Curve takes lower side but not cut the passivation curve indicate very little resistance to localized corrosion, (formation biggerloop),NT= Not Tested for 654SMO and 654SMO in cases of measurements with chelant.

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Figure 1: Electrochemical measurement (OCP, Tafel plots, anodic and cyclic polarization.) representative curves of different testmetals in PAA bleach media without and with chelant, (values of Cl¯ are in ppm).

4. ResultsCorrosion rate was calculated by weight loss formula (1); weight loss is determined by subtracting the weight of thecorroded and cleaned coupon from its original weight. The extent of pitting (evaluated by measuring the maximumdepth of pits formed on freely exposed surface), and crevice corrosion on occluded surface formed by serrated washers.Values of these parameters are given in Table 3. The tests did not show any remarkable pitting attack on stainless steelscontaining 6% Mo. in test media without chloride.

The potentiostatic test was conducted to evaluate pitting potential Ep, which decides the vertex potential required forrecording the cyclic polarization curves, The closed loop cyclic curves in Figure 1 indicate that localized corrosion hasbegun but was subsequently repassivated and that the downward curve therefore shows a weighted average ofalloys behaviour in the solution and the localized corrosion environment. Some open loop curves (figure 1) indicate thatthe pits have not repassivated. Corrosion parameters Ecorr, Epass, Ep, Ec and CR are obtained from these tests aregiven in Table 4.

OCP 654SMO OCP 317L in PAA

Anodic Pol. Curves. 316L Anodic, 316L with MgSO4

Cyclic curves 316L

Tafel Plot 254

Anodic Pol. 254SMO

Cyclic Pol curves 654SMO Cyclic Pol curves 2205 with EDTA

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5. Discussion5.1. Corrosivity of solutionsThe corrosion rate and pits’ depth of stainless steels were found greater in case of PAA solution without chelant PAA incomparison to solutions containing chelant. So, corrosivity of media reduces on adding chelant. These values aremaximum for all the test metals in PA3 solutions and minimum for PA1CI i.e. in PAA solution with chelant EDTA(Table 3). No, accountable pit is observed for solutions of PAA with chelant for metals 6% Mo. On adding chloride intest solutions, corrosion rate and maximum pits depth is observed as increasing which was highest in PA3 solution andlowest in PA1 solution this behaviour of test samples indicates that corrosivity of stainless steels increases with Cl¯concentration into the test media. Similarly, comparison of these parameters (Table 3) on adding chelants in test mediashow PA2CII to be more corrosive than PA1CI and PA2CII is also found more corrosive than PA2CI we conclude thatsolutions having more chloride were more corrosive. The solutions with MgSO4 were more corrosive than solutionswith EDTA for all the three Cl¯ levels. Corrosivity in peracetic media was found according to chloride concentrationPA1<PA2<PA3 and according to chelant PACI<PACII.

Test results from Tabel plots (Table 4) indicate that value of Ecorr of test metals is highest in PA1 and decreases onadding chloride while increases again when measured on adding chelant. This improvement was more in case of chelantEDTA i.e. in solution PA1CI. On increasing Cl¯, the passivation range (Ep-Ecorr) decreases, this is expected due to Ep(pitting potential) decreases and Ecorr shifts to anodic direction. The values of OCP, Ep, Ec and Epass for tested SSshow that solutions PA3 is more corrosive than PA2 and PA2 is more corrosive than PA1CI.

According to ascending order of corrosivity, the solution can be put as;

Without chelant: PA1<PA2<PA3, with chelant: PA1CI<PA1CII<PA2CI<PA2CII< PA3CI<PA3CII

The greater corrosivity of PAA can be explained on the basis of (i) the prevailing reduction reaction which in turn affectthe corrosion of materials (ii) the nature of peracetic acid. The following reduction can be considered to be responsiblefor corrosion in the solutions.

CH3COOO¯ +3H++ 3e¯ ↔ CH3COOOH + H2O (2)

According to this reduction reaction, two ions of CH3COOO¯ are responsible for consumption of 6 electrons. Thuscorrosion of steels takes place when exposed to peracetic acid media , Peracetic acid is more oxidizing than hydrogenperoxide due to electrophilicity character which is greater in case of PAA is than another peracid such as Caro’s acid so,at a given pH PAA is also more corrosive than Caro’s acid [11]. Results (Table 4) show that the open circuit potential ofdifferent steels is higher than respective Ecorr an increase in chloride content decrease in passivation range in bothcases without and with chelant.

Margin of safety, MOS i.e. (Passivation range) was found as reduced with corrosivity of solutions for test metals thisrate was found lower in PAA solutions with chelant. A reduction in Ep is observed in same manner in case ofpotentiostatic and cyclic tests. Values of Ep, Ec and Epass is found greatest for 654SMO, 254SMO intermediate for SS-2205 and lowest for 317L and 316L while for 316L a rapid reduction is observed with chloride concentration asincreasing in Cl¯ makes this SS more vulnerable to localized corrosion.

No remarkable pitting was observed under microscope [20X] for 6% Mo SS in test solutions containing chelantswithout Cl¯. Some anodic and cyclic polarization Curves within test solutions show a kink in the curves (Fig. 1 anodic& cyclic) which indicates onset of pitting on higher potential due to O2 evolution reaction. In some cases, cyclicpolarization curve shows hysteresis behaviour when polarization scan reverses to cathodic direction on reaching vertexpotential.

5.2. Material performance5.2.1. Weight Loss Test:

In peracetic acid bleach solutions, on the basis of weight loss the metals can be put in following increasing order ofobserved corrosion resistance (Tables 2).Without Cl¯: 316L<317L<2205<254SMO<654SMO,With Cl¯=500 ppm: 316L<317L≤2205 <254SMO<654SMO,With Cl¯=1000 ppm: 316L≤317L≈ 2205<254SMO<654SMO.

(i) SS-316L and 317L have lowest resistance in all. It can be explained on the basis of composition i.e. its lower PRENo. (PRE No =%Cr + 3:3 × %Mo + 16 × %N) 23.89 and 29.20 respectively as well as content of carbon isgreater than metals having 6% Mo. Performance of metal 317L is better than 316L due to it contains more quantity

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of Mo hence greater *PRE No. than 316L, as concentration of chloride increased, corrosion rate of 316L is seenhighest, increased rapidly than 317L. Duplex SS-2205 exhibits intermediate performance as per its composition i.e.greater PRE. No. than 316L and 317L but its corrosion rate improved rapidly in presence of extended Cl¯. So, Incase of PA3 solution 2205 does not show good performance for handling in PAA bleach media.

(ii) The metals 654SMO & 254SMO show best resistance against all types of corrosion. CR and pits depth are lowestfor these metals due to having highest PRE No. The 654SMO showed better performance than 254SMO due tolarger Cr and Mo value in composition. With increasing chloride concentration CR increased by lower rate incomparison to SS-316L & 317L. So, we concluded that 654SMO & 254SMO metals can be handled in peraceticacid bleach media with extended value of chloride.

5.2.2. Electrochemical tests

(i) On the basis of anodic polarization measured parameters OCP, Ep, Ecorr & Epass (Table 4), this is found that valueof Ep is highest & of Ecorr The metals 6% Mo SS (654SMO, 254SMO) showed higher Ep and lower Ecorr valuesi.e. greater Epass (passivation range) in comparison to SS-2205, 317L and 316L. In cyclic polarization curves(Fig.1), Ep can be seen very close to Ec i.e. smaller value of (Ep-Ec), we can say smaller cyclic loop forms forthese metals, which implies highest resistance against localized corrosion of 6% Mo SS as having higher amountof Cr, Mo, Ni and N in its composition and highest PRE No., CCT, CPT values. .

(ii) SS-316L shows very low value of above parameters due to containing lowest quantity of Cr, Mo and nitrogen inits composition. This metal contains greater value of carbon which is also responsible for its weak performanceamong the tested metal samples.

(iii) Duplex SS-2205 shows intermediate behaviour by corrosion parameters in different test solutions with lowerchloride or non chloride, but in case of extended chloride contained solution (PA3), the behaviour of SS-2205tends to SS-317L and 316L. This is due to its weak resistance against chloride environmental bleach media.

(iv) In case of corrosion measurements with chelant, however, resistance against crevice corrosion is observed to behigher in case of PA1CI, PA1CII (solutions non Cl¯, with chelant) than in solution PA1 (without chelant).Passivation range and pitting potential Ep is highest for 6% Mo SS and lowest for 316L. Results suggest higherresistance of 2205 than 317L and much higher than 316L against localized.

(vi) Cyclic polarization curves show non passivation pits (Ec<Ecorr) especially in case of 316L and 317L in chloridecontained solutions and their Ec is likely to be around Ecorr. For metal 2205, Ep and hence passivation range(Epass) is measured as intermediate among the five tested metals in non chelant case. Ec>Ecorr behaviour of SS-654SMO and 254SMO showed repassivation of pits and more resistance to localized corrosion in all the PAAsolutions. SS-2205 also exhibited good performance but under the condition of nil or lower quantity of chloridewith chelant in PAA media.

(v) The stainless steel 2205 in PA2, PA3 (PAA with Cl¯) solutions showed a lower value of Ep, Ec & Epass whichindicates smaller resistance to crevice corrosion. But on adding chelant, the values of Ep and Epass improvedespecially in case of EDTA inhibitor. So, the performance of 2205 is still poorer with chloride. While relativeperformance of SS-316L, 317L is found poorer with and without chloride which improved on adding chelant.

(vi) Metal 254SMO, 654SMO show best resistance against pitting and crevice corrosion in solutions PA2 & PA3 too.Due to having highest PRE No. , these can be suggested suitable to handle in PAA solutions with chloride. Thestainless steels 317Land 316L are not appropriate to handle in PAA bleach media with extended chloride.

5.3. Effect of chloride(i) On adding chloride, CR increased with concentration of chloride in all the test solutions of PAA bleach media. On

adding NaCl, Na+ and Cl¯ ions become free as charge careers in solution by which conductivity of solutionincreases. According to penetration theory [7, 22] anodic dissolution take place by chloride i.e. Cl¯ ions replacethe oxygen from per acetate ion by which protective layer becomes weaker on attacking Cl¯ ion on the surface ofmetal. So reaction rate increases, metal oxidation rate will also increase due to this is electrochemical process. So,corrosion of SS in peracetic acid bleach media accelerates by increasing concentration of NaCl.

(ii) As concentration of chloride increases, the behaviour of SS-2205 is seen same as 317L in the solution PA3 thisexhibits good performance in PA1 or PA2CI (Cl¯ =500 ppm with EDTA). SS-654SMO, 254SMO showed bestperformance (highest corrosion resistance) in all test solutions, while SS-316L is found most affected metal undereach condition of solutions in all the tested stainless steels.

The results through EC measurements show a reduction in value of Ep and displacement of Ecorr in anodicdirection (anodic as well as cyclic tests), hence a reduction in Epass i.e. in (margin of safety) MOS is found in

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case of all the test metals in similar test conditions. An improvement in resistance against corrosion was seen inbleach media with chelant for all the stainless steels.

(iii) The values of OCP, Ep, Epass & Ec are seen least affected for 6% Mo SS, while for 2205 metal more affected thanSS-317L in PA2 and PA3 solutions as this metal have higher PRE No. CCT & CPT. for SS-2205 and 316Lvalue of these parameters (Table 4) are found more affected when concentration of chloride extended above 500ppm. So, this metal did not show good performance in the solutions with extending limit of chloride.

(iv) For SS 316L, Cyclic polarization measurements show lowest Epass and its' Ec tends to Ecorr when chlorideconcentration increased in solution. Value of Ep-Ec is bigger as a result a big loop forms which indicates lowestresistance to localised corrosion.

5.4. Effect of chelantA surface film formed by reaction of EDTA and metal cations as complex materials at the metal solution interfaceconsidered as barrier which is responsible to inhibition,

2Fe++ + [EDTA] → Fe [EDTA]Such reactions are possible for other metal composition of SS. The absorbed inhibitor EDTA or MgSO4 used in PAAmedia may form a surface film that acts as a physical barrier to restrict the diffusion of ions or molecules to or from themetal surface and so retard the rate of corrosion reactions hence works as inhibitor.

The results from weight loss test were analyzed on adding chelant in test solutions as per Table 2. The values of CR aswell as pits depth decreased for all the test conducted metals in every condition of chemical charges (Table 3). This isdue to passive layer on metal surface becomes strong on adding chelant.

With reference to electrochemical tests, on adding chelant, the corrosivity of solutions is seen as reduced and hencevalue of Ecorr decreased and corresponding value of pitting potential Ep increased in both the anodic and cyclicpolarization curves; as a result the MOS value (margin of safety) increased for all the test metals without and with Clˉ.The value of OCP increases in presence of chelant EDTA or MgSO4 in PAA solutions. This effect indicates that the testmaterials can be handled in media with EDTA having chloride (Table 4). The values of Ep & Ec for metal 2205 and317Lwere found greater in solutions with chelant EDTA than 316L. This improvement was greater in case of EDTAthan MgSO4. SS-2205 showed best performance with chelant EDTA in case of smaller quantity of chloride in the bleachsolutions. Inhibition efficiency is observed more in case of EDTA than MgSO4.

5.5. EDTA versus MgSO4 inhibitor

As per analyzed earlier the corrosion parameters show that bleach solutions with chelant MgSO4 is more corrosive thansolutions with chelant EDTA for both without and with chloride in test media. Varjonen et al [21] studied on titanium inperoxide and found that MgSO4 was not found effective inhibitor.

Experimental results imply that EDTA is better inhibitor than MgSO4 as SO4¯ ¯ ion make complex as single site butEDTA has six coordination sites free with four negative ions on its four branches on which it make complex. Thereduction reaction with EDTA takes place rapidly [15]. EDTA has stronger ability to complexion. It has COOOH andNH2 functional group which form a ring structure around the metal. So, we can handle the more affective metals or withextended limit of Cl¯ in the peracids with EDTA inhibitor.

5.6. Material SelectionA proposition is now given about the appropriate material for handling against adopted PAA bleach solutionsconsidering the results and analysis of immersion test, criterion by Tuthill [20] and EC polarization measurements.With reference to material selection we concluded as follows:

(i) The stainless steels 654SMO, 254SMO are appropriate in PA1, PA2 and PA3 solution at pH=4.0. Theirsuitability remains same on adding the chloride while 317L and SS-2205 are not appropriate for PA3 and PA2solutions under the similar conditions. Their use is doubtful in view of the electrochemical measurements.Immersion test showed better performance of 2205 in PA1 and much better with chelant in same solutions.Exhibition of resistance of 254SMO & 654SMO against localized corrosion indicates that these steel to be mostappropriate in PAA beach solutions.

(ii) SS-2205 appears usable in the solutions PA1 & PA2 but with chelant on the basis of all types of tests. The 316Lis not appropriate in PA2 and PA3 solutions its use is doubtful in view of the electrochemical test measurements.Immersion test showed intermediate performance of 2205 in PA1 this performance again improves in PA1CI i.e.Performance improves on adding chelant in same solutions. SS-317L shows better performance than 316L, Thiscan be handled in presence of chelant having lower limit of chloride, this behaviour of 317L is similar to SS-2205.

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(iii) Duplex stainless steel 2205 shows even better resistance in these solutions with chelant which is acceptable forPA2CI and PA2CII too, at room temperature. It is may be more appropriate in solutions with lower values (<500ppm) of chloride but with EDTA inhibitor. The metals 654SMO and 254SMO are suitable for handling inperacetic acid solutions containing extending limit of chloride (Tables 2 & 3). Application of 2205 is more costeffective on the basis of its improved cost/strength ratio in addition to better corrosion resistance and can beconsidered suitable for handling with EDTA inhibitor with extended limit of chloride. This aspect of corrosionbehaviour of this metal needs further investigation.

6. Conclusion

Present study reports corrosion investigation carried out on selected stainless steels (Table1) in laboratory preparedperacetic acid solutions with/without chloride and chelant at pH=4. Following conclusions can be drawn on the basis ofanalysis of corrosion test results.

(i) Corrosion rate and maximum pits depth increases with test metals according to654SMO<254SMO<2205<317L<316L. The observed increasing order of corrosivity of test solutions can be putas PA1<PA2<PA3, and with chelant: PA1CI<PA1CII<PA2CI<PA2CII< PA3CI<PA3CII i.e. corrosivity ofsolutions increases on adding chloride while decreases on adding chelant.

(ii) The value of OCP of all the test material also decreases according to above manner with chloride in the above testsolutions with and without chelant. The 654SMO, 254SMO show highest OCP value while SS-2205 showsintermediate value and 317L, 316L show lowest for similar conditions (Table 4).

(iii) Electrochemical measurements were run on stainless steels in PAA solutions with Cl¯ exhibit higher degree oflocalized corrosion whereas it is less severe in case of solutions without chloride. Corrosion parameters obtainedfrom electrochemical tests i.e. OCP, Ep, Ec, Epass etc. (Table 4) indicate that test metals can be put in followingorder of increasing corrosion resistance 316L<317L<2205 <254SMO<654SMO in test media. Performance ofstudied stainless steels with chelants EDTA & MgSO4 is seen as improved against corrosion attack. The measuredcorrosion parameters imply that corrosion resistance is greater in case of solutions with EDTA than solutions withMgSO4 under similar conditions i.e. corrosivity of media reduced on adding chelants for all the tested stainlesssteels in the present investigations.

(iv) On the bases of long term immersion and electrochemical corrosion measurements in PAA, it is concluded thatmetal 654SMO and 254SMO show highest resistance against all types of corrosion. The comparison ofperformance of different test materials suggests, SS-2205 to be resistant in case of nil chloride PAA media while316L least resistant against all types of corrosion attacks.

(v) In PAA solutions with chelants, the suitability of test metals is fixed with extending limit of chloride. The SS-2205can be handled in bleach media with chloride as well as with chelant MgSO4 and found more suitable in mediawith EDTA. In this case this metal can be handled containing even by increased quantity of chloride in PAAsolutions,

(vi) This is also concluded that 654SMO & 254SMO are apt for handling in extended Cl¯ containing PAA media. SS-2205 exhibits greater corrosion resistance than 317L in case of solutions with chelant while SS-316L showslowest. This behavior is due to its greater PRE No. of 2205. So, this metal is appropriate for handling in bleachsolutions without Cl¯ or with lower concentration of chloride on the basis of degree of attack alongsidecomparison of cost versus strength of different stainless steels.

AcknowledgementFor experimental facilities the Corrosion Laboratory, Department of Paper Technology I.I.T. Roorkee, India isthankfully acknowledged.

References

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Stockholm, Sweden, p. 133.[13] Singh R. and Singh A. K., “Corrosion studies of stainless steels in peroxide bleach media. Tappi.J. Dec. 1995.Vol. 78 No. 12

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corrosion study of stainless steels in peracetic acid bleach media

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